The present invention relates to an aromatic polycarbonate and a production process therefor. More specifically, it relates to a process for producing an aromatic polycarbonate from an aromatic dihydroxy compound and a carbonic acid diester, both having a low content of an aldehyde compound, by an ester exchange method and to an aromatic polycarbonate having excellent color and stability obtained by the above process.
Polycarbonates which are superior to other resins in moldability, mechanical strength and optical properties such as achromatic transparency are widely used as materials for transparent substrates for recording media which record and/or reproduce information using laser light, such as audio disks, laser disks, optical disk memories and magneto-optic disks, as well as for transparent sheets and lenses.
The polycarbonates are produced from an aromatic dihydroxy compound and a carbonic bond forming precursor. As the production process thereof are known an interfacial polycondensation process in which phosgene is directly reacted as the carbonate bond forming precursor and a melt polycondensation process in which an ester exchange reaction between a carbonic acid diester and phosgene is carried out. The melt polycondensation process has such an advantage that a polycarbonate resin can be produced at a lower cost than the interfacial polycondensation process.
A polycarbonate produced by an ester exchange melt polycondensation process using a conventionally known ester exchange catalyst such as an alkali metal salt catalyst, for example, sodium hydroxide is disclosed in xe2x80x9cPlastic Material Course (17) Polycarbonate, Chapter 4, pp. 48-53xe2x80x9d published by Nikkan Kogyo Shimbun Co., Ltd. Since this polycarbonate is obtained by polymerization by distilling off a monomer component such as a phenol, an aromatic dihydroxy compound or diphenyl carbonate at a temperature of 250xc2x0 C. or more for 1 hour or more, undesired side reactions such as branching or decomposition occur during this polymerization. These undesired side reactions include a decarboxylation reaction and a Kolbe-Schmitt similar reaction described in xe2x80x9cChemistry and Physics of Poly-carbonates, pp. 47-48xe2x80x9d written by H. Schnell and published by Interscience Publishers Co., Ltd. When these side reactions occur, color developing impurities or a branch structure are formed in the obtained polycarbonate and the obtained polycarbonate is apt to deteriorate in color and to become inferior in heat resistance and hydrolysis resistance as it contains a hetero-bond component other than its own carbonate bond in the molecule, or in homogeneity and transparency as it contains a gelled substance.
Therefore, the application of a polycarbonate produced by the melt polymerization process is restricted compared with a polycarbonate produced by the interfacial polymerization process.
To solve the above problems, paying attention to metal impurities contained in a carbonic acid diester and/or an aromatic dihydroxy compound which are raw materials for the production of an aromatic polycarbonate, there are proposed a method for reducing the content of an element such as Na, Fe, Cr or Mn (refer to JP-A 5-148355 and JP-A 6-32885) (the term xe2x80x9cJP-Axe2x80x9d as used herein means an xe2x80x9cunexamined published Japanese patent applicationxe2x80x9d) and a method for reducing the amount of an inorganic non-metal ion such as hydrolysable chlorine (JP-A-2-153927).
Meanwhile, as for organic impurities contained in the carbonic acid dieter and/or aromatic dihydroxy compound, it is well known that organic impurities having a benzene ring are contained in the aromatic dihydroxy compound such as 2,2-bis(4-hydroxyphenyl)propane (to be referred to as xe2x80x9cbisphenol Axe2x80x9d hereinafter) as disclosed in xe2x80x9cHigh Purification Technology System, Vol. 3, High-purity Substance Production Process (published by Fuji Technosystem), pp. 149-160, 1997xe2x80x9d and documents cited in the above document.
It is also known that a carbonate bond forming precursor, for example, a carbonic acid diester may contain an impurity having a salicylic acid structure which is a product of a decomposition reaction similar to a Kolbe-Schmitt reaction, or an impurity having a benzophenone skeleton.
It is also proposed to solve the above problems by controlling the amounts of organic impurities and the above metallic or inorganic ionic impurities contained in bisphenol A or carbonic acid diester (EP-A 872507 and JP-A 7-33866). However, it cannot be said that the obtained polycarbonate is satisfactory in terms of color and stability. That is, it cannot be said that problems with color and stability are completely solved for industrial-scale production.
Further, as proposals aimed to solve these problems using an ester exchange catalyst, JP-A 4-89824 discloses a catalyst which comprises 1) a nitrogen-containing basic compound, alkali metal compound and boric acid or boric acid ester, JP-A 4-46928 discloses a catalyst which comprises an electron donating amine compound and alkali metal compound, and JP-A 4-175368 discloses a technology for adding an acidic compound and epoxy compound to a polycarbonate produced by melt polycondensation in the presence of an alkali metal catalyst.
However, the problems with color and stability are not completely solved by the above conventional methods such as control of the amounts of impurities or selection of the type of catalyst.
It is an object of the present invention to provide a process for producing an aromatic polycarbonate which is excellent in color and stability by an ester exchange method.
It is another object of the present invention to provide a process for producing an aromatic polycarbonate which is excellent in color and stability and has a low content of a hetero-bond such as a branch structure.
It is still another object of the present invention to provide a process for producing an aromatic polycarbonate which is particularly excellent in color with a conventionally unknown minus b value.
It is a further object of the present invention to provide an aromatic polycarbonate which is excellent in both color and stability as described above.
Other objects and advantages of the present invention will become obvious from the following description.
According to the present invention, firstly, the above objects and advantages of the present invention are attained by a process for producing a polycarbonate by melt polycondensing a dihydroxy compound and a carbonic acid diester in the presence of an ester exchange catalyst, wherein a raw material which contains a dihydroxy compound represented by the following formula (1): 
wherein R1 and R2 are each independently an alkyl group having 1 to 20 carbon atoms, alkoxy group having 1 to 20 carbon atoms, cycloalkyl group having 6 to 20 carbon atoms, aryl group having 6 to 20 carbon atoms, cycloalkoxy group having 6 to 20 carbon atoms or aryloxy group having 6 to 20 carbon atoms, m and n are each independently an integer of 0 to 4, and X is a single bond, oxygen atom, carbonyl group, alkylene group having 1 to 20 carbon atoms, alkylidene group having 2 to 20 carbon atoms, cycloalkylene group having 6 to 20 carbon atoms, cycloalkylidene group having 6 to 20 carbon atoms, arylene group having 6 to 20 carbon atoms or a group having 6 to 20 carbon atoms, and an aldehyde compound in an amount of no more than 3xc3x9710xe2x88x926 equivalent in terms of an aldehyde group based on 1 mol of the dihydroxy compound represented by the above formula (1) is used as one raw material comprising the above dihydroxy compound and a raw material which contains a carbonic acid diester and an aldehyde compound in an amount of no more than 3xc3x9710xe2x88x926 equivalent in terms of an aldehyde group based on 1 mol of the carbonic acid diester is used as the other raw material comprising the above carbonic acid diester.
According to the present invention, secondly, the above objects and advantages of the present invention are attained by an aromatic polycarbonate pellet which comprises an aromatic polycarbonate composed mainly of a recurring unit represented by the following formula (2): 
wherein R1 and R2 are each independently an alkyl group having 1 to 20 carbon atoms, alkoxy group having 1 to 20 carbon atoms, cycloalkyl group having 6 to 20 carbon atoms, aryl group having 6 to 20 carbon atoms, cycloalkoxy group having 6 to 20 carbon atoms or aryloxy group having 6 to 20 carbon atoms, m and n are each independently an integer of 0 to 4, and X is a single bond, oxygen atom, carbonyl group, alkylene group having 1 to 20 carbon atoms, alkylidene group having 2 to 20 carbon atoms, cycloalkylene group having 6 to 20 carbon atoms, cycloalkylidene group having 6 to 20 carbon atoms, arylene group having 6 to 20 carbon atoms or alkylene-arylene-alkylene group having 6 to 20 carbon atoms, and having a viscosity average molecular weight of 10,000 to 17,000 and a value of 1xc3x9710xe2x88x926 to 20xc3x9710xe2x88x926 obtained by dividing the average value of absorbance at a wavelength of 400 nm and absorbance at a wavelength of 430 nm by absorbance at a wavelength of 260 nm, and which has a b value of xe2x88x921.0 to 0.0 measured in accordance with JIS K7105.
The present invention will be described hereinbelow. A description is first given of the process of the present invention.
In the present invention, the one raw material comprising a dihydroxy compound contains a dihydroxy compound represented by the formula (1) and an aldehyde compound in an amount of no more than 3xc3x9710xe2x88x926 equivalent, preferably no more than 2xc3x9710xe2x88x926 equivalent, more preferably no more than 1xc3x9710xe2x88x926 equivalent in terms of an aldehyde group based on 1 mol of the dihydroxy compound represented by the formula (1). This raw material having a low content of an aldehyde compound can be advantageously prepared by contact hydrogenating a raw material containing an aldehyde compound in an amount of more than 3xc3x9710xe2x88x926 equivalent in terms of an aldehyde group based on 1 mol of the dihydroxy compound represented by the formula (1).
In the present invention, the other raw material comprising a carbonic acid diester contains a carbonic acid diester and an aldehyde compound in an amount of no more than 3xc3x9710xe2x88x926 equivalent, preferably no more than 2xc3x9710xe2x88x926 equivalent, more preferably no more than 1xc3x9710xe2x88x926 equivalent in terms of an aldehyde group based on 1 mol of the carbonic acid diester. The other raw material having a low content of an aldehyde compound can be advantageously prepared by contact hydrogenating a raw material containing an aldehyde compound in an amount of more than 3xc3x9710xe2x88x926 equivalent in terms of an aldehyde group based on 1 mol of the carbonic acid diester.
In the present invention, the total amount of aldehyde compounds contained in the one raw material comprising a dihydroxy compound and the other raw material comprising a carbonic acid diester is preferably no more than 3xc3x9710xe2x88x926 equivalent, more preferably no more than 2xc3x9710xe2x88x926 equivalent, particularly preferably 1xc3x9710xe2x88x926 equivalent in terms of an aldehyde group based on 1 mol of the dihydroxy compound represented by the above formula (1).
A contact hydrogenation reaction for the one raw material comprising a dihydroxy compound and the other raw material comprising a carbonic acid diester can be carried out on a mixture of the both materials.
The contact hydrogenation reaction is preferably carried out in a reaction solvent in the presence of a catalyst.
Preferred examples of the catalyst used for contact hydrogenation include conventionally known heterogeneous catalysts such as palladium-carbon, platinum-carbon, palladium black and ruthenium-carbon.
The reaction solvent used at the time of contact hydrogenation is preferably a lower alcohol in the case of the aromatic dihydroxy compound and an ether-based solvent in the case of the carbonic acid diester. Illustrative examples of the lower alcohol include methanol, ethanol and isopropyl alcohol, and illustrative examples of the ether-based solvent include tetrahydrofuran, dioxane and ethylene glycol dimethyl ether.
The content of the aldehyde group can be easily reduced to 3 xcexc-equivalents or less based on 1 mol of the dihydroxy compound by contact hydrogenation. Preferably, the solvent contains no aldehyde compound. Further, to control the total content of specific metal components in the aromatic dihydroxy compound and the carbonic acid diester to a low level, a solvent for application in the electronic industry having a low total content of specific metal impurities is more preferred.
The content of the aldehyde group is measured by a fluorescent derivation technique described in xe2x80x9cBUNSEKI KAGAKU Vol. 34, pp. 314-318, 1985xe2x80x9d. The aldehyde group detection limit of the technique is 0.5xc3x9710xe2x88x926 equivalent/mol or less.
The above aldehyde compound include aliphatic aldehydes such as formaldehyde, acetaldehyde and hexylaldehyde, alicyclic aldehydes and aromatic aldehydes.
The aldehyde compound is often contained in the dihydroxy compound and the carbonic acid diester as an impurity.
After contact hydrogenation, a conventionally known purification method such as cleaning, recrystallization, crystallization, sublimation purification or distillation is preferably carried out. A combination thereof is particularly recommended.
In the present invention, the one raw material contains a carboxylic acid compound in an amount of preferably no more than 3xc3x9710xe2x88x926 equivalent, more preferably no more than 2xc3x9710xe2x88x926 equivalent, particularly preferably no more than 1xc3x9710xe2x88x926 equivalent in terms of a carboxyl group based on 1 mol of the dihydroxy compound represented by the above formula (1).
The carboxylic acid compound includes lower carboxylic acids such as formic acid, acetic acid, propionic acid, oxalic acid, glycolic acid, malic acid, citric acid and tartaric acid. The content of the carboxylic acid compound which is the above upper limit or less can be attained by a purification method such as cleaning after the above contact hydrogenation.
Examples of the dihydroxy compound represented by the above formula (1) include BPA, bis(4-hydroxyphenyl)methane, bis(4-hydroxy-3,5-dimetylphenyl)methane, 1,1-bis(4-hydroxyphenyl)ethane, 1,1-bis(4-hydroxyphenyl)-1-phenylethane, 1,1-bis(4-hydroxyphenyl)propane, 2,2-bis(2-hydroxyphenyl)propane, 2,2-bis(4-hydroxy-3-methylphenyl)propane, 2,2-bis(4-hydroxy-3,5-dimethylphenyl)propane, 2,2-bis(4-hydroxy-3,5-dibromophenyl)propane, 2,2-bis(3-isopropyl-4-hydroxyphenyl)propane, 2,2-bis(4-hydroxy-3-methylphenyl)propane, 2,2-bis(4-hydroxy-3-phenylphenyl)propane, 2,2-bis(4-hydroxyphenyl)butane, 2,2-bis(4-hydroxyphenyl)-3-methylbutane, 2,2-bis(4-hydroxyphenyl)-3,3-dimethylbutane, 2,4-bis(4-hydroxyphenyl)-2-methylbutane, 2,2-bis(4-hydroxyphenyl)pentane, 2,2-bis(4-hydroxyphenyl)-4-methylpentane, 3,3-bis(4-hydroxyphenyl)pentane, 1,1-bis(4-hydroxyphenyl)cyclohexane, 1,1-bis(4-hydroxyphenyl)-3,3,5-trimethylcyclohexane, 9,9-bis(4-hydroxyphenyl)fluorene, 9,9-bis(4-hydroxy-3-methylphenyl)fluorene, bis(4-hydroxyphenyl)sulfide, bis(4-hydroxyphenyl)sulfone, 4,4xe2x80x2-dihydroxydiphenyl sulfoxide, 4,4xe2x80x2-dihydroxydiphenyl sulfide, 4,4xe2x80x2-dihydroxydiphenyl ketone, 4,4xe2x80x2-dihydroxydiphenyl ether, hydroquinone, 2-t-butylhydroquinone, resorcin, 4,4xe2x80x2-dihydroxydiphenyl and those having an alkyl group or aryl group as a substituent in the aromatic ring thereof. Out of these, BPA is particularly preferred from an economical point of view. They may be used alone or in combination of two or more.
The one raw material comprising a dihydroxy compound is used in the form of globular particles which include particles having a diameter of 0.1 to 3 mm in an amount of 70 wt % or more and have a specific surface area of 0.05 to 0.2m2/g and a pore volume of 0.01 to 0.1 ml/g. As for particle size, particles having a diameter of 0.1 to 2 mm are contained in an amount of preferably 70 wt % or more, more preferably 80 wt % or more, particularly preferably 90 wt % or more. Further, particles having a diameter of 0.1 mm or less are contained in an amount of preferably 10 wt % or less, more preferably 5 wt % or less, particularly preferably 3 wt % or less.
The specific surface area measured by a BET method is preferably 0.05 to 0.1 m2/g.
The pore volume measured by a mercury penetration method (obtained based on the assumption that a peak at a radius of 100 nm to 600 xcexcm is regarded as a particle pore distribution) is preferably 0.01 to 0.6 ml/g, more preferably 0.01 to 0.4 ml/g, particularly preferably 0.01 to 0.03 ml/g.
An aromatic polycarbonate which is excellent in color and transparency and has small variations can be obtained by using the above one raw material comprising a dihydroxy compound as globular particles having the above particle diameter distribution, specific surface area and pore distribution.
The L and b color values of the above globular particles are preferably 80 or more and 2 or less, more preferably 83 or more and 1.5 or less, much more preferably 85 or more and 1 or less, particularly preferably 85 or more and 0.5 or less, respectively.
The carbonic acid diester is a carbonic acid diester of an aryl group having 6 to 10 carbon atoms, aralkyl group or alkyl group having 1 to 4 carbon atoms, which may be substituted. Specific examples of the carbonic acid diester include diphenyl carbonate, ditolyl carbonate, bis(chlorophenyl)carbonate, m-cresyl carbonate, bis(diphenyl)carbonate, diethyl carbonate and dibutyl carbonate. Out of these, diphenyl carbonate is preferred.
The above other raw material comprising a carbonic acid diester preferably has a small specific surface area and a small pore volume like the above one raw material comprising a dihydroxy compound. However, as the carbonic acid diester has higher stability than the dihydroxy compound, the necessity of controlling the particle size distribution is smaller than the dihydroxy compound. Since the raw material is often supplied as a solution, the necessity of taking into consideration the specific surface area and pore volume is small.
The reason why the above effect is developed when the dihydroxy compound is prepared in the form of globular particles as described above is unknown but it is assumed that quality deteriorating factors are adhered to the surface of each particle and taken into the inside of each pore of the dihydroxy compound, thereby deteriorating the quality of the dihydroxy compound.
In the present invention, in consideration of influence on the durability, color and transparency of a polycarbonate to be produced, it is recommended to use raw materials having a total content of trace metal elements including transition metal elements such as Fe, Cr, Mn, Ni, Pb, Cu and Pd, metal and metalloid elements such as Al and Ti of preferably 50 ppb or less, more preferably 10 ppb or less, in addition to the controlled contents of the above aldehyde group and carboxyl group as impurities.
To obtain an aromatic polycarbonate having higher durability, it is preferred that the one raw material comprising a dihydroxy compound and the other raw material comprising a carbonic acid diester should contain an alkali metal element and/or an alkali earth metal element having large ester exchange capability in an amount of only 60 ppb.
To obtain an aromatic polycarbonate having much higher durability, it is more preferred that the content of an alkali metal element and/or an alkali earth metal element in the aromatic dihydroxy compound and carbonate bond forming precursor should be no more than 60 ppb and that the total content of transition metal elements in the above compound and the precursor should be no more than 10 ppb.
Further, the total content of the above metals and metalloid elements in the both raw materials is preferably no more than 20 ppb.
Although the total content of the transition metal elements, metals and metalloid elements in the raw materials is preferably as small as possible, the limits of the conventional technologies are more than 10 ppb. An aromatic polycarbonate having excellent durability can be obtained by using a dihydroxy compound and a carbonic acid diester having a total content of the above elements of no more than 10 ppb.
In the present invention, to obtain a raw material comprising a dihydroxy compound which has a reduced total content of transition metal, metal and metalloid element impurities and a raw material comprising a carbonic acid diester which has also a reduced total content of the above impurities, known purification methods such as distillation, extraction, recrystallization and sublimation may be employed. It is more preferred to combine the above purification methods.
To obtain a polycarbonate having a low total content of metal impurities in the present invention, a high-purity solvent having an extremely low total content of metal impurities, for example, a solvent for use in the electronic industry is preferably used for the purification of the raw materials.
In the process of the present invention, the above raw materials are used to produce a polycarbonate by melt polycondensing a dihydroxy compound and a carbonic acid diester in the presence of an ester exchange catalyst.
The ester exchange catalyst is preferably a combination of a) at least one basic compound selected from the group consisting of a nitrogen-containing basic compound and a phosphorus-containing basic compound and b) at least one metal compound selected from the group consisting of an alkali metal compound and an alkali earth metal compound.
The alkali metal compound or alkali earth metal compound is, for example, a hydroxide, hydrocarbon compound, carbonate, carboxylate such as acetate, stearate or benzoate, nitrate, nitrite, sulfite, cyanate, thiocyanate, borohydride, hydrogenphosphate, hypophosphite, bisphenol or phenol salt of an alkali metal or alkali earth metal.
Specific examples of the alkali metal compound and alkali earth metal compound include sodium hydroxide, potassium bicarbonate, sodium carbonate, potassium carbonate, cesium carbonate, lithium acetate, rubidium nitrate, lithium nitrate, sodium nitrite, sodium sulfite, sodium cyanate, potassium cyanate, sodium thiocyanate, potassium thiocyanate, cesium thiocyanate, sodium stearate, sodium borohydride, potassium borohydride, lithium borohydride, sodium phenylborate, sodium benzoate, disodium hydrogenphosphate, dipotassium hydrogenphosphate, lithium hypophosphite, sodium hypophosphite, potassium hypophosphite, rubidium hypophosphite, cesium hypophosphite, barium hypophosphite, cesium hypophosphite, disodium salts, monopotassium salts, sodium potassium salts of bisphenol A and potassium salts of phenol.
Out of these, the alkali metal compound is preferably a hypophisphite of an alkali metal and the alkali earth metal compound is preferably a hypophosphite of an alkali earth metal.
The above metal compound is preferably used in an amount of 5xc3x9710xe2x88x928 to 5xc3x9710xe2x88x926 equivalent based on 1 mol of the dihydroxy compound.
In the present invention, (a) an alkali metal salt of an ate complex of a group XIV element of the periodic table or (b) an alkali metal salt of an oxoacid of a group XIV element of the periodic table disclosed in JP-A 7-268091 may be used as the above alkali metal compound used as a catalyst. The group XIV element of the periodic table is silicon, germanium or tin.
By using the alkali metal compound as a polycondensation reaction catalyst, a polycondensation reaction can proceed quickly and completely. Also, undesirable side reactions such as a branching reaction which proceeds during a polycondensation reaction can be suppressed to a low level.
At least one compound selected from the group consisting of an oxoacid and oxide of a group XIV element of the periodic table and an alkoxide and phenoxide of the same element may be optionally used as a cocatalyst in combination with the above catalyst in the polycondensation reaction of the present invention.
Undesirable phenomena such as a branching reaction liable to occur during a polycondensation reaction, a main chain cleavage reaction and the generation of foreign matter or burn mark in a molding apparatus during molding can be effectively suppressed without ill-affecting the terminal blocking reaction and polycondensation reaction rate by using the cocatalyst in a specific ratio, which is preferred for the object of the present invention.
Examples of the oxoacid of the group XIV element of the periodic table include silicic acid, stannic acid and germanic acid.
Examples of the oxide of the group XIV element of the periodic table include silicon dioxide, tin dioxide, germanium dioxide, silicon tetramethoxide, silicon tetraphenoxide, tetraethoxy tin, tetranonyloxy tin, tetraphenoxy tin, tetrabutoxy germanium, tetraphenoxy germanium and condensates thereof.
The cocatalyst is preferably used in an amount of 50 molar atoms or less in terms of the group XIV element of the periodic table based on 1 molar atom-of the alkali metal element contained in the polycondensation reaction catalyst. When the cocatalyst is used in an amount of more than 50 molar atoms in terms of the metal element, the polycondensation reaction rate slows down disadvantageously.
The cocatalyst is more preferably used in an amount of 0.1 to 30 molar atoms in terms of the group XIV element of the periodic table as the cocatalyst based on 1 molar atom of the alkali metal element contained in the polycondensation reaction catalyst.
As the catalyst are used a nitrogen-containing basic compound and a phosphorus-containing basic compound. These compounds may be used alone or in combination of two or more.
Examples of the nitrogen-containing basic compound include ammonium hydroxides having an alkyl, aryl or alkylaryl group such as tetramethylammonium hydroxide, tetrabutylammonium hydroxide and benzyltrimethylammonium hydroxide; basic ammonium salts having an alkyl, aryl or alkylaryl group such as tetramethylammonium acetate, tetraethylammonium phenoxide, tetrabutylammonium carbonates and benzyltrimethylammonium benzoates; tertiary amines such as triethylamine and dimethylbenzylamine; and basic salts such as tetramethylammonium borohydride, tetrabutylammonium borohydride and tetramethylammonium tetraphenylborate.
Examples of the phosphorus-containing basic compound include phosphonium hydroxides having an alkyl, aryl or alkylaryl group such as tetrabutylphosphonium hydroxide and benzyltrimethylphosphonium hydroxide; and basic salts such as tetramethylphosphonium borohydride, tetrabutylphosphonium borohydride and tetramethylphosphonium tetraphenylborate.
The above nitrogen-containing basic compound or phosphorus-containing basic compound is preferably used in an amount of 1xc3x9710xe2x88x925 to 1xc3x9710xe2x88x923 equivalent in terms of the basic nitrogen atom or basic phosphorus atom based on 1 mol of the dihydroxy compound. The amount is more preferably 2xc3x9710xe2x88x925 to 5xc3x9710xe2x88x924 equivalent, particularly preferably 5xc3x9710xe2x88x925 to 5xc3x9710xe2x88x924 equivalent based on the same standard.
It has been found that in order to improve the color of the obtained polycarbonate at this point, use of the nitrogen-containing basic compound or phosphorus-containing basic compound in such an amount that it does not exceed 20xc3x97Fe*)+200 xcexc-equivalent (Fe* (wtppb): the total content of iron contained in the dihydroxy compound and the carbonic acid diester as raw materials) is particularly effective. The amount is particularly preferably such that it does not exceed 20xc3x97(Fe*)+150.
Although the reason is not made clear, it is presumed that the color of a polycarbonate is worsened by interaction between iron contained in the nitrogen-containing basic compound or phosphorus-containing basic compound as a raw material and the nitrogen-containing basic compound and/or phosphorus-containing basic compound. From this point of view, it is preferred to reduce the total content of metal impurities as much as possible.
The melt polymerization process is carried out by stirring a dihydroxy compound and a carbonic acid diester under normal pressure and/or a vacuum nitrogen atmosphere while they are heated and distilling off the formed alcohol or phenol. The reaction temperature, which differs according to the boiling point of the formed product or the like, is generally 120 to 350xc2x0 C. to remove the alcohol or phenol formed by the reaction.
The formed alcohol or phenol is easily distilled off by placing the system under vacuum in the latter stage of the reaction. The inside pressure of the system in the latter stage of the reaction is preferably 133.3 Pa (1 mmHg) or less, more preferably 66.7 Pa (0.5 mmHg) or less.
In melt polymerization, another copolymerizable compound given below may be optionally incorporated in the main chain of a polycarbonate in addition to the above dihydroxy compound (aromatic dihydroxy compound) and carbonic acid diester.
Examples of the copolymerizable compound include aliphatic and alicyclic diols and polyols such as ethylene glycol, 1,4-butanediol, polyethylene glycol, 1,4-cyclohexane dimethanol, 3,9-bis(1,1-dimethyl-2-hydroxyethyl)-2,4,8,10-tetraoxaspiro[5.5]undecane, tricyclo(5.2.1.02,6)decanedimethanol, trimethylolpropane and pentaerythritol; aromatic polyhydroxy compounds such as 1,1,1-tris(4-hydroxyphenyl)ethane and 1,1,2,2-tetrakis(3-methy-4-hydroxyphenyl)ethane; aliphatic and aromatic oxycarboxylic acids such as lactic acid, parahydroxybenzoic acid and 6-hydroxy-2-naphthoic acid; dicarboxylic acids such as succinic acid, fumaric acid, adipic acid, dodecane diacid, terephthalic acid, 2,6-naphthalenedicarboxylic acid, pyromellitic acid and trimellitic acid; and polycarboxylic acids.
To produce a polycarbonate by carrying out a reaction by melt polycondensation, a terminal capping agent and an antioxidant agent such as steric hindrance phenol may be used as required. The polycarbonate of the present invention includes branched polycarbonates prepared by copolymerizing a polyfunctional aromatic compound having a functionality of 3 or more and polyester carbonates prepared by copolymerizing an aromatic or aliphatic bifunctional carboxylic acid. Two or more of the obtained polycarbonates may be mixed together.
The molecular weight of the polycarbonate is preferably 10,000 to 22,000, more preferably 12,000 to 20,000, particularly preferably 13,000 to 18,000 in terms of viscosity average molecular weight (M) as a substrate material. A polycarbonate having the above viscosity average molecular weight has sufficiently high strength as an optical material and excellent melt fluidity at the time of molding and is therefore free from molding strain. For transparent applications such as sheets, the viscosity average molecular weight of the polycarbonate is preferably selected from a range of 17,000 to 100,000, more preferably from a range of 20,000 to 80,000.
According to the present invention, as a polycarbonate particularly suitable for the production of an optical disk substrate, there is provided an aromatic polycarbonate pellet which comprises an aromatic polycarbonate composed mainly of a recurring unit represented by the following formula (2): 
wherein R1 and R2are each independently an alkyl group having 1 to 20 carbon atoms, alkoxy group having 1 to 20 carbon atoms, cycloalkyl group having 6 to 20 carbon atoms, aryl group having 6 to 20 carbon atoms, cycloalkoxy group having 6 to 20 carbon atoms or aryloxy group having 6 to 20 carbon atoms, m and n are each independently an integer of 0 to 4, and X is a single bond, oxygen atom, carbonyl group, alkylene group having 1 to 20 carbon atoms, alkylidene group having 2 to 20 carbon atoms, cycloalkylene group having 6 to 20 carbon atoms, cycloalkylidene group having 6 to 20 carbon atoms, arylene group having 6 to 20 carbon atoms or alkylene-arylene-alkylene group having 6 to 20 carbon atoms, and having a viscosity average molecular weight of 10,000 to 17,000 and a value of 1xc3x9710xe2x88x926 to 20xc3x9710xe2x88x926 obtained by dividing the average value of absorbance at a wavelength of 400 nm and absorbance at a wavelength of 430 nm by absorbance at a wavelength of 260 nm, and which has a b value of xe2x88x921.0 to 0.0, preferably xe2x88x920.5 to 0.0 measured in accordance with JIS K7105.
In the present invention, to obtain an aromatic polycarbonate which hardly experiences a reduction in molecular weight and discoloration, attention is paid to the viscosity stability of a molten polymer. The melt viscosity stability is evaluated by the absolute value of a change in melt viscosity measured at a shear rate of 1 rad/sec in a stream of nitrogen at 300xc2x0 C. for 30 minutes and expressed by a change rate per minute. This value is preferably 0.5% or less. When this value is large, the deterioration of the polycarbonate by hydrolysis may be promoted. The present inventor judges that this value should be set to 0.5% to ensure the practical level of stability to hydrolysis. To this end, the melt viscosity is preferably stabilized by using a melt viscosity stabilizer after polymerization.
The melt viscosity stabilizer in the present invention also has the function of deactivating part or all of the activity of a polymerization catalyst used for the production of a polycarbonate.
As for the addition of the melt viscosity stabilizer, for example, it may be added while the polymer is molten after polymerization or after the polycarbonate is pelletized and re-molten. In the former case, the melt viscosity stabilizer may be added while the polycarbonate which is the reaction product in the reactor or extruder is molten, or may be added and kneaded while the polycarbonate obtained after polymerization is pelletized from the reactor through the extruder.
Any known melt viscosity stabilizer may be used. From the viewpoint of the large effect of improving the color and physical properties such as heat resistance and boiling water resistance of the obtained polymer, sulfonic acid compounds such as organic sulfonic acid salts, organic sulfonic acid esters, organic sulfonic anhydrides and organic sulfonic acid betain may be used, and phosphonium salts of sulfonic acid and/or ammonium salts of sulfonic acid are preferably used. Out of these, dodecylbenzenesulfonic acid tetrabutyl phosphonium salts and paratoluenesulfonic acid tetrabutyl ammonium salts are particularly preferred.
The above aromatic polycarbonate has an aryloxy group and a phenolic hydroxyl group as the main terminal groups and the concentration of the phenolic hydroxyl group is preferably 60 mol % or less, more preferably 40 mol % or less, particularly preferably 30 mol % or less. When the phenolic terminal group is contained in the above weight ratio, the object of the present invention can be more advantageously attained and the moldability of the composition (mold staining properties, releasability; to be simply referred to as xe2x80x9cmoldabilityxe2x80x9d hereinafter) is also improved.
The aryloxy group is preferably a phenyloxy group having a hydrocarbon group having 1 to 20 carbon atoms as a substituent, or nonsubstituted phenyloxy group. From the viewpoint of resin heat stability, a phenyloxy group having a tertiary alkyl group, tertiary aralkyl group or aryl group as a substituent, or nonsubstituted phenyloxy group is preferred.
Preferred examples of the aryloxy group include phenoxy group, 4-t-butylphenyloxy group, 4-t-amylphenyloxy group, 4-phenylphenyloxy group and 4-cumylphenyloxy group.
In the interfacial polymerization process, the concentration of the terminal phenolic hydroxyl group can be reduced to a low level by means of a molecular weight modifier. However, in the melt polymerization process, the concentration of the terminal hydroxyl group must be reduced positively because an aromatic polycarbonate containing a terminal phenolic hydroxyl group in an amount of generally 50 mol %, sometimes 60 mol % or more is readily produced through a chemical reaction.
That is, the following conventionally known method 1) or 2) can be advantageously used to adjust the concentration of the terminal hydroxyl group to the above range:
1) method of controlling the molar ratio of charge stocks; The molar ratio of the carbonic acid diester to the dihydroxy compound at the time of charging for a polymerization reaction is increased to a range of 1.01 to 1.10 in consideration of the characteristic features of a polymerization reactor.
2) terminal capping method; At the end of a polymerization reaction, the terminal hydroxyl groups are capped by adding a salicylate-based compound described in USP 5696222 in accordance with the method disclosed by the above document.
When the salicylate-based compound is used to cap the terminal hydroxyl groups, the amount of the salicylate-based compound is preferably 0.8 to 10 mols, more preferably 0.8 to 5 mols, particularly preferably 0.9 to 2 mols based on 1 chemical equivalent of the terminal hydroxyl group before a capping reaction. By adding the salicylate-based compound in the above weight ratio, 80% or more of the terminal hydroxyl groups can be capped advantageously. To carry out this capping reaction, catalysts disclosed by the above US patent are preferably used.
The concentration of the terminal hydroxyl group is preferably reduced before the deactivation of a polymerization catalyst.
Salicylate-based compounds enumerated in the specification of U.S. Pat. No. 5,696,222 may be preferably used as the salicylate-based compound, as exemplified by 2-methoxycarbonylphenylaryl carbonates such as 2-methoxycarbonylphenyl-phenyl carbonate; 2-methoxycarbonylphenyl-alkyl carbonates such as 2-methoxycarbonylphenyl-lauryl carbonate; 2-ethoxycarbonylphenyl-aryl carbonates such as 2-ethoxycarbonylphenyl-phenyl carbonate; 2-ethoxycarbonylphenyl-alkyl carbonates such as 2-ethoxycarbonylphenyl-octyl carbonate; (2xe2x80x2-methoxycarbonylphenyl)esters of aromatic carboxylic acids such as (2-methoxycarbonylphenyl)benzoate; and aliphatic carboxylates such as (2-methoxycarbonylphenyl)stearate and bis(2-methoxycarbonylphenyl)adipate.
An aromatic polycarbonate is obtained by the above process. When molded articles are formed from the aromatic polycarbonate, a conventionally known processing stabilizer, heat stabilizer, antioxidant, ultraviolet light absorber, antistatic agent, flame retardant and release agent may be added according to application purpose.
For example, various stabilizers may be blended to prevent a reduction in molecular weight and deterioration in color of an aromatic polycarbonate. Examples of the heat stabilizer include hypophosphorous acid, phosphorous acid, phosphoric acid, phosphonous acid, phosphonic acid, salts and esters thereof, steric hindrance amine antioxidants, steric hindrance phenolic antioxidants and carbon radical scavengers. Trisnonylphenyl phosphate, tris(2,4-di-t-butylphenyl)phosphite, tetrabutylphosphonium dihydrogenphosphates, tetrakis(2,4-di-t-butylphenyl) 4,4xe2x80x2-biphenylenediphosphinate, trimethylphosphate, dimethyl benzenephosphonate, 5,7-di-t-butyl-3-(3,4-dimethylphenyl)-3H-benzofuran-2-one, n-octadecyl 3-(3,5-di-t-butyl-4-hydroxyphenyl)propionate and 2-t-butyl-6-(3-t-butyl-2-hydroxy-5-methylbenzyl)-4-methylphenyl acrylate are preferably used. These heat stabilizers may be used alone or in combination of two or more. The amount of the heat stabilizer is preferably 0.0001 to 1 part by weight, more preferably 0.0002 to 0.5 part by weight, particularly preferably 0.0005 to 0.1 part by weight based on 100 parts by weight of the aromatic polycarbonate.
The aromatic polycarbonate may be mixed with a release agent to further improve releasability from a mold at the time of melt molding. Examples of the release agent include olefin-based wax, olefin-based wax containing a carboxyl group and/or carboxylic acid anhydride group, silicone oil, organopolysiloxane, higher fatty acid ester of a monohydric or polyhydric alcohol, paraffin wax and beeswax. The amount of the release agent is preferably 0.01 to 5 parts by weight based on 100 parts by weight of the aromatic polycarbonate.
The higher fatty acid ester is preferably a partial ester or whole ester of a monohydric or polyhydric alcohol having 1 to 20 carbon atoms and a saturated fatty acid having 10 to 30 carbon atoms. The partial ester or whole ester of a monohydric or polyhydric alcohol and a saturated fatty acid is preferably monoglyceride stearate, triglyceride stearate or pentaerythritol tetrastearate. The amount of the release agent is preferably 0.01 to 5 parts by weight based on 100 parts by weight of the aromatic polycarbonate.
The aromatic polycarbonate may be mixed with inorganic and organic fillers to improve its stiffness. Examples of the inorganic filler include lamellar or granular inorganic fillers such as talc, mica, glass flake, glass bead, calcium carbonate and titanium oxide, and fibrous fillers such as glass fiber, glass milled fiber, wollastonite, carbon fiber, aramide fiber and metal-based conductive fiber, and examples of the organic filler include organic particles such as crosslinked acryl particle and crosslinked silicone particle. The total amount of the inorganic and organic fillers is preferably 1 to 150 parts by weight, more preferably 3 to 100 parts by weight based on 100 parts by weight of the aromatic polycarbonate.
The above inorganic fillers usable in the present invention may be surface treated with a silane coupling agent. A favorable effect such as the suppression of the decomposition of the aromatic polycarbonate is obtained from this surface treatment.
The aromatic polycarbonate may further be blended with another resin.
Examples of the another resin include polyamide resins, polyimide resins, polyether imide resins, polyurethane resins, polyphenylene ether resins, polyphenylene sulfide resins, polysulfone resins, polyolefin resins such as polyethylene and polypropylene, polyester resins such as polyethylene terephthalate and polybutylene terephthalate, amorphous polyarylate resins, polystyrene resins, acrylonitrile/styrene copolymer (AS resin), acrylonitrile/butadiene/styrene copolymer (ABS resin), polymethacrylate resins, phenol resins and epoxy resins.
The aromatic polycarbonate obtained in the present invention has the effect of retaining color and durability, particularly durability for a long time under extreme temperature and humidity conditions. Substrates, obtained from the polymer, for high-density optical disks typified by CD, CD-ROM, CD-R, CD-RW, magnetic optical disks (MO) and disk versatile disks (such as DVD-ROM, DVD-Video, DVD-Audio, DVD-R and DVD-RAM) can retain high reliability for a long time. The aromatic polycarbonate is particularly useful for high-density optical disks such as digital versatile disks.
Sheets formed from the aromatic polycarbonate produced in the present invention are excellent in adhesion and printability and widely used in electric parts, building material parts and auto parts thanks to the above characteristic properties. More specifically, they are useful for optical applications such as various window materials, that is, grazing products for window materials for general houses, gyms, baseball domes and vehicles (such as construction machinery, automobiles, buses, bullet trains and electric vehicles), various side wall panels (such as sky domes, top lights, arcades, wainscots for condominiums and side panels on roads), window materials for vehicles, displays and touch panels for OA equipment, membrane switches, photo covers, polycarbonate resin laminate panels for water tanks, front panels and Fresnel lenses for projection TVs and plasma displays, optical cards, liquid crystal cells consisting of an optical disk and a polarizer, and phase difference compensators. The thickness of the sheet does not need to be particularly limited but it is generally 0.1 to 10 mm, preferably 0.2 to 8 mm, particularly preferably 0.2 to 3 mm. Various treatments for providing new functions (such as a laminate treatment for improving weatherability, a treatment for improving scratch resistance to improve surface hardness, surface drawing and processing for making translucent or opaque) may be carried out on the aromatic polycarbonate sheet.
To mix the above components with the aromatic polycarbonate, any means is employed. For example, a tumbler, twin-cylinder mixer, super mixer, Nauter mixer, Banbury mixer, kneading roll or extruder is advantageously used. The thus obtained aromatic polycarbonate resin composition is melt extruded to form a sheet directly or after it is pelletized by a melt extruder.
In an extrusion step (pelletizing step) for obtaining palletizing polycarbonate resin to be injection molded, foreign matter is preferably removed by passing the polycarbonate through a sintered metal filter having a filtration accuracy of 10 xcexcm while it is molten. An additive such as a phosphorus-based antioxidant is preferably added as required. Anyway, it is desired to reduce the contents of foreign matter, impurities and solvent in the raw material resin as much as possible before injection molding.
To produce an optical disk substrate from the above polycarbonate resin, an injection molding machine (including an injection compression molding machine) is used. This injection molding machine may be a generally used injection molding machine but preferably an injection molding machine whose cylinders and screws are made from a material having low adhesion to a resin and corrosion resistance and wearing properties in order to suppress the formation of a carbide and improve the reliability of a disk substrate. The preferred injection molding conditions include a cylinder temperature of 300 to 400xc2x0 C. and a mold temperature of 50 to 140xc2x0 C., thereby making it possible to obtain an optical disk substrate having excellent optical properties. The molding environment is preferably as clean as possible in consideration of the object of the present invention. It is also important that the material to be molded should be completely dried to remove its water and that retention which causes the decomposition of a molten resin should be prevented.
The aromatic polycarbonate produced in the present invention may be used for any purpose, for example, electronic and communication equipment, OA equipment, optical parts such as lenses, prisms, optical disk substrates and optical fibers, electronic and electric appliances such as home electric appliances, lighting members and heavy electric members, mechanical materials such as car interior and exterior parts, precision machines and insulating materials, miscellaneous materials such as medical materials, safety and protective materials, sports and leisure outfits, and home supplies, container and package materials, and display and decoration materials. They may also be advantageously used as a composite material with another resin, or organic or inorganic material.